Microlenses are widely used in various fields including an optical information processing system, an optical communication, an optical pickup, an optical measurement, or solid-sate image devices. Solid-state image devices typically include a photosensor such as a photodiode formed in or on a substrate, a color filter formed over the photosensitive device and a microlens array formed over the color filter. The photosensor may be a photodiode, a CMOS (complimentary metal oxide semiconductor) sensor or a charge-coupled device (CCD), for example. The function of the microlens is to efficiently collect incident light falling within the acceptance cone and refract this light in an image formation process onto a focal plane at a depth defined by the planar array of photodiode elements. In particular, development of more precise and small-sized microlenses is recently accelerated owing to miniaturization, integration and high performance requirements to optical instruments.
One conventional method for manufacturing the microlens uses a thermal process and a blanket etch-back process to form a dielectric layer as a microlens array on a substrate, but has difficulties in controlling uniformity, profile and curvature of the microlenses. FIGS. 1A to 1D show conventional ladder etching process sequences for forming a microlens array. In FIG. 1A, a substrate 10 includes a silicon nitride layer 12 on which discrete sections of photoresist pattern are formed through the use of photolithography. The photoresist pattern is then thermally reflowed to produce rounded discrete photoresist sections 14. The ladder etching process sequence is then carried out to produce ladder structures 12a as shown in FIG. 1B. After an ashing process for removing the remainder of the photoresist, a chemical downstream etching (CDE) process is employed to smooth the surface of the ladder structures 12a till curved microlenses 12b are created on the substrate 10 as shown in FIG. 1C. This ladder etching process sequences, however, only provides a lens height of 2˜6K Angstroms, causing a problem of insufficient curvature. If the amount of photoresist pull back and the ladder height are altered for controlling thickness and slope of the microlens to reach a lens height of more than 6K Angstroms, an undesired UFO-shaped microlens 12c will occur as depicted in FIG. 1D. Also, the remainder of the photoresist on the ladder profile is not uniform, and thereby the conventional microlens structure usually accompanies a roughness issue. In addition, a unique tool is requested for the CDE process, which needs long process time and has low throughput, causing problems in the mass production.
Accordingly, a novel method is needed for the image device fabrication to produce a microlens array with desired curvature, height and profile by using a simple etching process.